A combined service-brake and spring-loaded brake cylinder of the general type under consideration is known from DE 29 36 899 C2, for example. In this known actuator, the service brake cylinder and the spring-loaded brake cylinder are combined to form a structural unit and are separated from one another by a partition wall. A piston is arranged in an axially movable manner within the spring-loaded brake cylinder, and a storage spring rests against one side of the piston. By means of its other axial end, the storage spring is supported against the bottom of the spring-loaded brake cylinder. An inlet opens into a pressure chamber formed between the partition wall and the piston in the spring-loaded brake cylinder. Via the inlet, a pressure medium can be introduced into the pressure chamber to move the piston in the direction of the bottom of the spring-loaded brake cylinder, compressing the storage spring in the process. If, on the other hand, the pressure in the pressure chamber of the spring-loaded brake cylinder between the partition wall and the piston is relieved upon actuation of a valve connected to the inlet, the storage spring moves the piston in the direction of the partition wall. The piston is connected to a piston rod, which extends axially through the partition wall into the region of a pressure chamber of the service brake cylinder. A seal inserted into the partition wall seals off the pressure chamber of the spring-loaded brake cylinder with respect to the piston rod. At its forward axial end extending into the region of a pressure chamber of the service brake, the piston rod carries a pressure piece. An inlet opens into the pressure chamber of the service brake. The inlet permits a pressure medium, in particular compressed air, to be introduced to actuate the service brake. This compressed air acts on a diaphragm, which is inserted within the service brake cylinder. A pressure piece in the form of a piston rod head is arranged on the side that faces away from the pressure chamber of the service brake. The piston rod head is connected to a piston rod, which extends axially out of the service brake cylinder and is connected to a brake actuating mechanism. If the service brake is actuated by introducing compressed air into the inlet, the compressed air moves the diaphragm, the piston rod head and the piston rod of the service brake such that the brake actuating parts connected to the piston rod come into effect and apply the brake of the motor vehicle. During this normal service braking, the spring-loaded parking brake remains in the inactive position, in which the storage spring remains compressed by maintenance of the pressure in the associated pressure chamber. The spring-loaded parking brake can come into effect when the service brake fails, that is, when the pressure medium connection into the inlet is interrupted or when it is to be used as an immobilization brake and acts as a parking brake when the vehicle is parked.
This known combined service-brake and spring-loaded brake cylinder is illustrated in FIG. 1 of DE 29 36 899 C2. It is apparent that this figure is an accurate drawing of the actuator and should therefore be regarded as substantially to scale. It follows from this that the maximum possible stroke of the piston of the spring-loaded brake cylinder and the maximum possible stroke of the piston of the service brake cylinder are substantially equal, and, as a result, the dimensions of the service-brake and spring-loaded brake cylinder in the longitudinal extent thereof are fixed.
In the case of known combined service-brake and spring-loaded brake cylinders, the stroke lengths of the service brake and of the spring-loaded brake are identical because the primary focus is on the apparent necessity of providing the same axial stroke for the service-brake cylinder and the spring-loaded brake cylinder in order to be able to actuate the vehicle brake with the necessary force both while driving and in a parking brake situation.